The ampersand is a ligature of E and T. If you look at it, you can see how it looks like a (lowercase) E and a T.

Despite the obvious conclusion that this is somehow related to ET, the frightening, horror-iblific alien bent on worldwide destruction, it actually is a condensed form of "et". "et" is the Latin word for "and".

So, when you use an ampersand, you're showing how scholarly you are and demonstrating your '3l33t hax0r' knowledge of Latin.